Micro Electro-Mechanical System (MEMS) inertial sensors, including accelerometers, gyroscopes and magnetometers, are generally well-known and commercially available from Honeywell International, Incorporated, the assignee of the present patent application. MEMS inertial sensor device mechanisms are formed of a thin (20-200 um) layer of silicon mounted on a borosilicate glass or silicon die. In turn, the MEMS sensor die are typically mounted in a separate package substrate formed of a disparate material, such as ceramic or plastic. These disparate package materials can have coefficients of thermal expansion (CTE's) twice to many times that of the glass or silicon die material. Because die mounting is necessarily accomplished at a high temperature, stresses on the die during cooling to room temperature cause the package to squeeze the die, which results in bias and other performance irregularities and may even cause irreparable damage. Often in operation the device mechanism will also experience stresses due to vibration and shock, which can also cause damage, even catastrophic damage.
MEMS inertial sensor device mechanisms are created using reactive ion etching (RIE) or deep reactive ion etching (DRIE) which both etch very small features through an entire mechanism layer. These processes permit the creation of intricately shaped structures in the x-y plane of the thin mechanism layer, but do not permit control in the z direction other than depth, rather akin to an intricate cookie cutter. The device mechanism may be attached to the die at any point or cut free to move in response to external inputs.
Isolation of MEMS inertial sensor device mechanisms from the package substrate has typically taken the form of isolation materials beneath the mechanism die with the isolation material having a CTE near that of the die or the substrate, or between the CTE's of the die and the substrate.
FIG. 1 and FIG. 2 are top and side views, respectively, that illustrate the isolation mechanism typical of the prior art. In a conventional MEMS capacitance readout inertial sensor device 1, mechanism die 2 are typically mounted “face up,” with a device mechanism 3 (or “mechanism device”, used interchangeably hereinafter) positioned on top of the die 2, i.e., opposite from the package substrate 4. The device mechanism 3 is spaced above the die 2 over capacitor plates 5 formed on the upper die surface 6. The device mechanism 3 is, by example and without limitation, attached through anchors 7 to the upper die surface 6. Electrically conductive paths or metallized traces 8 are led from the capacitor plates 5 and device mechanism 3 to wire bond pads 9 on the upper die surface 6.
The mechanism die 2 is attached to the package substrate 4, by example and without limitation, using gold, aluminum or solder stud bumps 11 between bond pads 13 and 14 formed respectively on the package substrate 4 and a lower die surface 15. Thereafter, electrical contact is made between the mechanism die 2 and the package substrate 4 by electrically conductive wires 16 bonded between the wire bond pads 9 and wire bond pads 17 formed on the package substrate 4.
Isolation of the MEMS inertial sensor device mechanism 1 from the package substrate 4 is typically provided by isolation material 18 positioned between the lower die surface 15 and an upper the surface 19 of substrate 4, the isolation material 18 having a CTE near that of either the die 2 or the substrate 4, or between the CTE's of the die 2 and the substrate 4.
While such isolation mechanisms based upon isolation materials interposed between the die and substrate are effective for many applications, they still represent a difference in CTE that may result in bias and other performance irregularities in sensitive devices.